312 able and lesser understood biological responses of trees. Many tree biomechanical models seek to describe a critical instance producing failure. The intent is to both organize and segregate the mechanical and the biological aspects of failure in order to explain the physics governing tree and wood behavior. Once defined, departures from model expectations can be helpful in addressing the variability associated with different species and individuals within species for specific situations. Four modeling challenges assume higher priority for open-grown trees. First, there are very few empirical data to describe the important mechani- cal parameters (drag coefficient, natural frequency, damping ratio, moduli of elasticity and rupture, etc.) necessary to build models to predict failure. (This is exacerbated by the logistic challenges of and available opportunities to test appropriate specimens.) Second, crown and root architecture of open-grown trees have not been adequately quantified. This is especially true given the inher- ent variability in crown and root architecture of trees growing in developed landscapes, where maintenance (e.g., cabling and pruning) and above- and belowground restrictions on growth add to the inherent variability. The complex crown and root architecture of amenity trees makes it dif- ficult to assume some of the mechanical param- eters necessary to assess the likelihood of failure, such as drag coefficient, crown area or volume, natural frequency, damping ratio, and critical over- turning moment. Third, maintenance practices and ontogenetic changes to relevant mechanical parameters (e.g., wood properties, defects, and crown and root architecture) undermine the appli- cation of simple models to assess the likelihood of failure beyond short timeframes. There are other aspects of the growth of open-grown trees (e.g., changes to wood properties and defects over time) that are similarly difficult to assume because they are inherently variable and have not been adequately quantified. Fourth, wind flow around open-grown trees in developed landscapes is com- plicated and has not been adequately quantified. Modeling approaches are desired, speak- ing to the difference between the biological understanding of trees and their informed risk management. An urban forest manager seeks a discrete number of measures to better commu- ©2014 International Society of Arboriculture Dahle et al.: Tree Biomechanics White Paper nicate and appraise the likelihood of failure. To do so, applied tree biomechanical models are often rooted in dynamics of movement result- ing from wind or water loads. These models often use static tests to appraise stability as a surrogate diagnostic measure. Such models can then be deployed for an individual tree, client property, or community for management pri- oritization and emergency planning. There is a research and development community focused on strategic applied modeling and measurement, yielding innovative tools and approaches. There also is a wide range of basic research modeling toward understanding community patterns from disturbances, natural acclimations, and ecologi- cal function gaining significance within a chang- ing climate marked by extreme weather patterns. There are many tools used in modeling research and field diagnostics that provide highly accurate data at a high rate of measurement. The pace at which newer, more precise tools enter the market often outstrips the critical testing and interpretation needed to gauge the usefulness of the data collected. There remains a lack of inte- grated research testing across regions, and effort can be characterized as localized activity defined by the commercial tools in use on limited species. Equipment and training costs govern the rate of tool adoption to provide generalized service or research capacity. Prohibitive tool costs often limit adoption to experienced consultants with special- ized training, as the tools are beyond the financial reach and therefore irrelevant to most practicing arborists. Additionally, there is limited efficacy in feeding highly refined tool-specific data into models where other parameters are roughly esti- mated. The opportunity does exist, however, to crowdsource data and aggregate existing public domain data streams to inform fully inte- grated models across species, regions, and time. FOCUS AREA 3: UNDERSTANDING THE MECHANISMS AND MODES OF FAILURE IN TREES Tree failure from wind, rain, ice, and snow is common in natural and urban landscapes. Typically, the stron- gest external loads that trees experience are caused by wind. When wind events are accompanied by precipitation, the likelihood of tree failure increases.
November 2014
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